1. Field of the Invention
The present invention relates to a spin transistor including a ferromagnetic source/drain and a semiconductor channel, and more particularly, to a spin transistor capable of changing a potential gradient of a channel into positive and negative values according to applied gate voltage and adjusting a spin-orbit coupling constant to a wide extent according to the applied gate voltage.
2. Description of the Related Art
As one of novel devices to overcome physical limits of conventional electronic devices, there have been ardent attempts to develop a spin transistor (or, Spin-FET). In 1990s, Datta and Das propose a spin transistor using a two-dimensional electron gas (2DEG) as a channel (see Applied physics letter, vol. 56, 665, 1990). Since then, there have been attempts to control the spin of spin-injected electrons, as well as to transfer spin-polarized electrons in a hybrid structure where a ferromagnetic and a semiconductor are incorporated to each other.
In order to perform a basic operation of a spin transistor using electron spin precession, spin-polarized electrons should be essentially injected from the ferromagnetic into semiconductor, and a gate electrode should be ultimately used to control the spin precession of the spin-polarized electrons injected into the 2-dimensional electron gas channel. In this case, a spin-orbit interaction in the 2-dimensional electron gas channel should be high in order to effectively control the spin precession, and a suitable electric resistance between the ferromagnetic and the semiconductor should be maintained to observe a spin injection signal in an electrical manner. The intensity of the spin-orbit interaction may be quantified by employing a spin-orbit coupling constant (α) that may be obtained by an SdH (Shubnikov de Haas) oscillation experiments. Up to date, it has been reported that a channel structure based on the InAs two-dimensional electron gas is composed of materials that best satisfy the above-mentioned requirements.
Silicon-based semiconductor technology has occupied the most important position in modern industries, and been designed and manufactured on the basis of MOSFET (Metal Semiconductor Field Effect Transistor). This conventional semiconductor-based transistor uses an electric field to control electric charges in the semiconductor. On the contrary, the spin transistor functions to control the electric charges and spin at the same time, and therefore may be used in switching elements, logic circuits and so on under the control of spin-polarized electrons. When there is an electric field (E) that is vertical to a wave vector (k) of electrons moving in the two-dimensional electron gas channel, a magnetic field is formed due to the spin-orbit interaction, as shown in the following equation: HRashba ∝ k×E. This is called a Rashba effect where a magnetic field induced by the spin-orbit interaction is formed in the y-axis direction when an electric current moves towards the x-axis direction and an electric field is applied towards the z-axis direction by means of the gate voltage. The spin-polarized electrons injected into the channel of the spin transistor may cause a spin precession through the magnetic field, and use the gate voltage to adjust a precession angle.
In order to supply an electric charge to the two-dimensional electron gas channel through which the source and drain electrodes of the spin transistor are connected to each other, a carrier supply layer may be formed in a lower side of the channel. In the case of the configuration of the conventional spin transistor where the carrier supply layer is formed in the lower side of the channel, however, an energy band structure and an electron distribution of the channel are highly asymmetric with being acutely inclined in one direction, and have a negative potential gradient. This potential gradient of the channel is changed into only a negative value within a limited range, regardless of the application of a gate voltage (or, the changes in applied voltage). This indicates that there are limits in embodying a spin transistor to improve the spin-orbit interaction.